Operation of thermionic tubes



June 18, 1935. PARSQNS 2,005,237

OPERATION OF THERMIONIC- TUBES Filed Feb. 2, 1929 L L I: 7 7

i L S w L y LF 28 /a g L92 7M 20 fl 1 1 1 z/ 1 1 bcde 29 gwuemtoz Patented June 18, 1935 UNITED STATES PATENT OFFICE Application February 2,

6 Claims.

This invention relates to my operation of thermionic tubes, and particularly in amplifying or receiving apparatus for radio signaling.

It is an object to provide an improved mode of operation and apparatus therefor including particularly the provision of an improved method and apparatus for actuating the filament and plate circuits of thermionic tubes.

A further purpose is to combine the improved method and apparatus just mentioned with an amplifier or receiver of radio signals.

A further purpose is generally to improve and simplify the method and apparatus for reception of radio signals.

The drawing shows a diagrammatic illustration of a circuit embodying the invention.

I, 2 and 3 are similar tubes of Well known construction including a filament, a grid and a plate. The grid of tube l is fed from an aerial or signal collector 4 through an adjustable tuning device. A signal received in the aerial 4 is amplifled in tube I, and the plate circuit thereof feeds the amplified signal to the grid of tube 2 through a transformer 6, to be again amplified in'tube 2, the plate circuit of which feeds the re-amplified signal to the grid of tube 3 through a transformerl.

Tube 3 is connected conventionally to act as a detector and rectified signals in the plate circuit thereof are passed through a signal indicating de-- vice 8 in this case of the telephone receiver type.

For actuating the filament and plate circuit current is derived from a relatively very low frequency source 9 which may be the ordinary house current mains at a frequency of 60 cycles. Such circuit is fed though a frequency changer, generally denoted by the numeral I0 and in this case consisting of a series of individual devices a-bc--de connected in series for each to receive current at a lower frequency and deliver current at a higher frequency. The individual devices af-b-cd--e may be of any suitable type but preferably are of a type having no moving parts, nor any parts consumed in the operation thereof, such for instance as a well known type known as the Joly static frequency transformer, consisting merely of magnetic cores and suitable current carrying coils. The Joly frequency changer triples the frequency whereby a series of five of these devices connected as described, will deliver an output frequency of approximately 15,000 cycles with the input frequency mentioned. Such output frequency is fed to a transformer generally denoted by the numeral ll, having a secondary .winding ll for delivering ahigh voltage, suitable 1929, Serial No. 336,945

for the greatest plate circuit requirement of the receiving set, and a secondary winding 3 for delivering a low voltage, suitable to operate the filament of all of the tubes of the set in parallel arrangement, as shown in the drawing.

A suitable relatively high resistance I4 is bridged across the terminals of the winding l3 and the electrical center thereof is connected to the ground. Relatively small capacities I6-|1 bridge the opposite sections of the resistance M to by-pass radio-frequency currents but to present relatively great resistance to the filament current of 15,000 cycles. A variable resistance l8, preferably non-inductive, controls the current to all the filaments.

The plate circuits of each of the amplifier tubes l--2 connects through the primaries of transformers 6-4 with the one terminal of the Winding I2 of transformer II. The other terminal connects with the filament circuit through a connection [9, whereby to complete the plate circuits through the current source and tube filament. A relatively high resistance 20, preferably non-inductive, is shunted across the terminals of winding l2 and the plate circuit of detector tube 8 is connected to a point 2| thereof which gives a potential less than the amplifier plate potential and suitable for the detector.

A continuous negative bias is given to the grid of amplifier tube I through a connection 22 with a low voltage battery 23 and a connection 24 to the mid-point of resistance l4, thereby also completing and grounding the aerial circuit.

The grid of amplifier tube 2 is given the same continuous negative bias through the same battery and a connection 25.

The grid of detector tube 3 is given a higher continuous negative bias by the means of a connection 26 through a battery 21 connected in series with the battery 23, the combined battery voltage being suflicient to operate the detector at the proper point in the characteristic output curve thereof.

To reduce the reactance of the alternating current output circuits of transformer H, such circuits are shunted by condensers 28-29 respectively, the capacity of the respective condensers being adjustable or variable over limits to insure that the total capacity and inductance of the circuit with which the condenser is associated will be such as to approximately tune the circuit to the frequency output of the frequency changer III.

In the operation of the device, during the period when the plates of the tubes are positive with respect to the filament, the action of the device will be substantially the same as if the plates were actuated by a corresponding direct current voltage. When the platesare negative with respect to the filament no plate current will flow and the device is inoperative. Also during a portion of the half cycle when the plate is positive the voltage will be too low to operate the tubes. Thus, during somewhat more than half the time, the efficiency of the device is zero or very low, making a low total efficiency unless means is provided to increase the efiiciency.

In an ordinary amplifier, operated by direct.

plate current, the efiiciency is limited by the tendency of a tube to oscillate when both the grid and plate circuits are tuned. Measures taken to prevent such oscillation introduce un avoidable losses whereby the total efficiency drops. It has been found that by operating the plate from alternating current of a frequency such as here proposed the normal tendency of a tube to oscillate is greatly decreased, whereby the losses mentioned are not necessary and the net or over-all result is at least equal to and may be greater than that obtained from direct current operation.

As one instance of measures taken'to prevent losses normally incurred with direct current operation, it may be noted that the primaries of transformers 61 are shown with, and may safely have, substantially one to one coupling with the tuned secondaries. With direct current plate operation such coupling must be relatively very small for receiving the higher frequency signals ordinarily used in broadcasting, or the tube will oscillate.

It appears probable that some regeneration effect existsin the device as shown in the drawing, during the positive-plate half cycle and brought about by the feed back from output to input circuits through the inter-element tube capacities, and through various inductive and capacitative couplings which normally exist.

But in any case such feed back is not normally sufiicientto cause tube oscillation. It is therefore possible to greatly reduce the resistance of all the various paths of the signal in the amplifiers, and to sharply tune all such paths whereby to attain a degree of signal selectivity otherwise only to be had by introducing or permitting sufficient regeneration that the tubes are on the verge of oscillation, which is objectionable because such a condition is not stable, and for other reasons.

It will be apparent, therefore, that important results flow from the application of this invention to a cascade amplifier in which a thermionic tubeis interposed between successive circuits tuned to the same signal frequency. In such an amplifier the intermittent application of effective positive potential to the plate of the tube with respect to its filament enables the tube to operate at peak efiiciency, while, due to the intervening period of low zero, or negative plate potential, it is impossible for the tube to break into oscillation, regardless of the extent to which the elements of loss ordinarily used to control or prevent oscillation are eliminated.

It will also be noted that it is broadly immaterial in the achievement of the effects described in the preceding paragraph whether the plate current is an alternating current or merely an intermittent direct current or a current which is continuous and direct but fluctuates from an effective value to a value so low as to check or eliminate feed back. In any case the result of changing or interrupting or reversing plate potential to the proper degree and in properly timed sequence in relation to the frequency of the signal amplified Will enable the amplifier to operate at peak efliciency, building up the signal intensity during the effective periods of maximum positive plate potential and then preventing oscillation by cutting off or reducing positive plate potential at or near the moment of breaking into oscillation.

Attention must be called furthermore to the fact that the application of proper frequencies and values of alternating or fluctuating current to the filament will, in itself, by changing filament potential'relatively to both plate and grid, have a similar effect in controlling oscillation, providing the frequency and value of potential are properly chosen. In any event, the frequency applied either to the filament or plate must be such that the ear will not detect either the frequency or the fact that the signal is intermittent and that its impulses endure for something less than one half the frequency period as above explained.

It is obviously impossible to state any particular values for the potential or frequency or rate and degree of fluctuation of current applied to filament or plate. In order to obtain a maximum exercise of the phenomenon of oscillation control herein described, it will be necessary to adapt these values to the characteristics of the tube and the frequency of the signal amplified. Where the latter frequency is low the amplifier will be relatively stable and the period of maximum plate potential or optimumfilament-grid relation may be relatively long. Where the amplifier is relatively unstable it is necessary to interrupt relatively frequently the conditions which make for maximum efficiency in the amplifier and hence tend to result in oscillation.

It may be noted that other frequencies may be used for the plate and filament circuits. It is important however that such frequencies should be sufficiently high to be substantiallyinaudible, and sufficiently low so that thereis a substantial difference between the current supply frequency and the signal frequency, whereby thesignal frequency may be by-passed through condensers such as [6-47 which are of a capacity too small to by-pass any substantial portion of the current supply frequency. It is also important. that the filament and plate supply frequency be the same, or at least if different, that they are such that no audible frequency'may be established by heterodyning their fundamental'or harmonic frequencies together or with the signal.

It is to be noted that, while all the tubes, including detector tube 3, are shown to be operated from the alternating current supply, this is not necessary and any tube or tubes, such for instance as detector 3, may be operated by direct current in the well known manner, or other well known types of detector, not requiring current in any form for their operation may be used, such for instance as a crystal detector.

I claim the following, and the equivalents thereof z 1. The method of operating a thermionic tube in a cascade amplifier having successive circuits tuned to pass the same frequency, which method consists in intermittently applying to the elements of the tube potentials tending to cause tube operation at efficiency so high as to tend to result in oscillation, and varying such potential intermittently upon one element of the tube at such intervals as to prevent oscillation.

2. The method of operating a thermionic tube in an amplifier having successive circuits tuned to pass a desired frequency, which method consists in intermittently applying to the elements of the tube relative potentials tending to cause tube operation at an efiiciency such as to destroy stability of operation if continued, and varying the relative potential between two such elements intermittently to the extent required to restore stability to the amplifier, the rate of such variation being such as to maintain the amplifier highly effective while preventing oscillation.

3. The method of operating a thermionic tube having input and output circuits substantially tuned to the same frequency, such method consisting in applying an alternating current potential between two elements of said tube of such value and frequency as to maintain said tube in operation at high efficiency for a part of the cycle of said current and to interrupt the operation of said tube at such efficiency before oscillation occurs.

4. In an amplifier, the combination of a thermionic tube having grid and plate circuits, one of which is tuned to the frequency of a desired signal, and a filament heating circuit, of a relatively low frequency source of alternating current, frequency changing means deriving power therefrom and having an output at inaudible frequencies, and a transformer connected with said frequency changer output and for delivery of inaudible frequencies and having separate secondaries respectively connected to the filament and plate circuits of said tube, whereby to supply both of said circuits from a common source with alternating current at identical inaudible frequencies and like phase.

5. In an amplifier in which oscillations other than signal oscillations at audible frequencies would be objectionable, the combination with a plurality of tuned circuits arranged to receive signal frequencies and subject to oscillation, of means for delivering a signal frequency to one of said circuits, means for receiving a signal from the other of said circuits, coupling means for said circuits including a thermionic tube having coupling elements, and means for periodically impressing across said elements an intermittent charge of adequate potential and periodically reducing the potential of said coupling charge at a super-audible rate and to a degree sufficient to avoid oscillation in either of the circuits coupled thereby, whereby said tube and circuits may intermittently operate with such efficiency that they would be subject to oscillation, but for said intermittent reduction of potential.

6. Signal amplifying apparatus comprising the combination with a series of successive circuits, of a coupling tube including a filament and plate, a source of low frequency alternating current, means for increasing the frequency of said current, a transformer primary coupled to receive the output of said frequency increasing means and separate transformer secondaries inductively connected with said primary and provided with connections respectively to the filament and plate of said tube whereby to energize said filament and plate simultaneously at their proper respective voltages and in like phase.

FRED A. PARSONS. 

